The invention relates to a method of manufacturing a polymer-dispersed liquid crystal cell, in which method a mixture, which predominantly comprises a liquid crystalline material as well as reactive monomers and a photoinitiator, is sandwiched between two substrates, which are provided with an electrode layer, whereafter the mixture is polymerized under the influence of radiation. The invention also relates to a polymerizable mixture which can suitably be used in a polymer-dispersed liquid crystal cell as well as on a display device comprising such a cell.
Polymer-dispersed liquid crystal cells (abbr. PDLC cells) are increasingly being used in electro-optic devices, such as display devices, optical projectors and electrically drivable optical shutters. The optically active material of these cells is formed by liquid crystalline material which is dispersed in a matrix of a polymerized material. Such a material is referred to as a polymer-dispersed liquid crystalline material (abbr. PDLC material). This material is customarily prepared by providing a mixture of a liquid crystalline material (70-95% by weight), reactive monomers (5-30% by weight) and at least one photoinitiator, in the form of a layer, between two substrates of a cell and, subsequently, polymerizing this layer under the influence of radiation. During polymerization, phase-separation occurs, which leads to the formation of the desired optically active layer of polymer-dispersed liquid crystalline material. This layer can be switched between an optically transparent state (in the presence of a field) and an optically scattering or translucent state (in the absence of a field) by means of an electric field.
A method of the type mentioned in the opening paragraph is known per se, for example, from European Patent publication EP-A 575.791. More particularly, in examples 5-18 of said publication, a description is given of a prepolymer of reactive monomers composed of 2-ethylhexylacrylate (EHA) as well as one or two polyfunctional monomers. One part by weight of this prepolymer is mixed with four parts by weight of a non-reactive liquid crystalline material. Also a small quantity of a photoinitiator is added to this mixture. The resultant polymerizable mixture is subsequently provided between two substrates and polymerized by means of UV light so as to form an optically active layer.
The known method has an important drawback. It has been found that the electro-optical response of the PDLC cells thus manufactured is not uniform at all parts of the surface of the cell. For example, the switching voltage necessary to switch from transparent to scattering, and vice versa, is found to be different for different parts made of PDLC material. It has further been found that the electro-optical properties of the PDLC material are insufficiently stable with respect to time. Life tests show that these properties deteriorate relatively rapidly. For example, the hysteresis and the switching voltage increase rapidly.